The present invention relates to seismic energy sources and, more particularly, to seismic energy sources configured to generate enhanced shear wave energy in order to more accurately determine the distribution and orientation of fractures in subterranean formations.
Seismic geophysical surveys, including microseismic monitoring, are often used in the oil and gas industry in order to map stratigraphy of subterranean formations, lateral continuity of geologic layers, locations of buried paleochannels, positions of faults in sedimentary layers, basement topography, and various other geographic structures. The resulting maps are typically deduced through analysis of the nature of reflections and refractions of generated seismic waves from interfaces between the multiple layers within the particular subterranean formation being mapped.
Microseismic activity generally emits elastic waves in the form of compressional waves (“p-waves”) and shear waves (“s-waves”). Microseismic detection can be utilized in conjunction with hydraulic fracturing or water flooding techniques in order to map created fractures. Microseismic detection is also commonly utilized in long term reservoir monitoring applications of either reservoir production or injection. A hydraulic fracture generates microseismic activity that emits p-waves and s-waves. The generated p and s-waves travel through the surrounding earth and are reflected by various subterranean formations to be detected by an adjacent detection system comprising, for example, an array of seismic detection devices. As the p and s-waves reach the detection system, the seismic detection devices transduce the p and s-waves into representative electrical signals. These electrical signals are processed to determine the locations of the microseisms in the reservoir. In order to obtain reliable microseismic data, it is necessary to obtain an accurate formation velocity model using artificial sources in known locations that will generate similar p and s-waves. These p and s-waves are also detected, transduced into electrical signals, and analyzed to determine the seismic nature of the subterranean formations at the given site.
Conventional seismic energy sources often utilize explosives, such as a perforation gun or a simple string shot having explosives wrapped thereabout. Typical perforation guns and string shots, however, provide explosive energy primarily in the radial direction, but fail to produce a substantial amount of seismic energy in the longitudinal direction. As a result, a large amount of p-wave energy is emitted into the surrounding formations, but little s-wave energy is generated. Since microseismic events typically exhibit large s-wave signatures as compared to the corresponding p-wave content, what is needed is a seismic energy source configured to generate substantial amounts of s-wave energy in order to calibrate a more accurate formation velocity model.